A cyanate ester polymer has a triazine ring formed by polymerization; and owing to its high heat resistance and excellent electric characteristics, the polymer is heretofore widely used as raw materials for various functional polymer materials such as structural composite materials, adhesives, electric insulating materials, electric/electronic parts, etc. However, with the recent requirement for advanced high-level performance in the field of these applications, the requirement for the properties of functional polymer materials has become severer and severer. The properties include, for example, flame retardance, heat resistance, low dielectric constant, low dielectric loss tangent, weather resistance, chemical resistance, low water absorbability, high fracture toughness, etc., but up to the present, these required properties are not always satisfied.
For example, in the field of printed circuit board materials, the materials are required to have a low dielectric constant and a low dielectric loss tangent with the current tendency toward higher frequency for communication frequency and clock frequency, and cyanate resins having excellent dielectric characteristics have become much used. In that situation, from the viewpoint of security of safety against fire, flame retardance must be imparted to them, and bromine compounds having high flame retardance are used. For example, known are bromobisphenol A (see Patent Reference 1), bromophenol-novolak glycidyl ether (see Patent Reference 2), bromomaleimides (see Patent Reference 3), halogen-having monofunctional cyanates (see Patent Reference 4), addition-type bromine compounds having no reactivity with cyanate ester compound (see Patent Reference 5). These bromine compounds have high flame retardance, but may have a risk of generating corrosive bromine or hydrogen bromide through thermal decomposition, and therefore, materials not containing a bromine-based flame retardant are desired.
Accordingly, phosphorus-containing compounds and nitrogen/sulfur-containing compounds are investigated as a flame retardance substitutable for bromine-based flame retardant. For example, as a phosphorus compound often incorporated in epoxy resin, investigated are triphenyl phosphate and resorcinol bis(diphenyl phosphate). However, when incorporated in large quantities, they often lower heat resistance, moisture resistance, water absorbability, etc. For overcoming it, known is a method of adding a phenolic hydroxyl group-having phosphorus compound to a cyanate compound (for example, see Patent References 6, 7, and 8); but the phosphorus compound is also problematic in point of its toxicity. As a nitrogen compound, used are melamine, guanidine, etc.; but when used alone, their flame retardance is insufficient.
On the other hand, a metal hydroxide such as aluminium hydroxide, magnesium hydroxide or the like may be mentioned as a flame retardant; but incorporation of a metal hydroxide may have a risk of lowering dielectric characteristics, heat resistance, impact resistance and moldability. In addition, for example, as in epoxy resin, use of a large amount of an inorganic filler such as spherical fused silica for reducing flammables and for securing flame retardance may take a risk of increasing the melt viscosity of molding materials, thereby lowering the moldability and the substrate wettability thereof to lower the adhesive power, and worsening the dielectric characteristics thereof.
Further, even an antimony-type flame retardant such as antimony trioxide, which is widely used as combined with a bromoepoxy resin, is also problematic in that it is a deleterious substance and therefore has a risk of chronic toxicity. From the above viewpoints, flame retardance of thermosetting resin itself is more desired than before.
For improving heat resistance, low dielectric constant, low dielectric loss tangent, weather resistance, chemical resistance, low water absorbability, high fracture toughness, moldability, adhesiveness and others simultaneously with flame retardance, many trials have heretofore been made. For example, disclosed are a method of producing a cured product having excellent thermal stability by combining a monocyanate and a dicyanate (see Patent Reference 9); and a method of attaining a low dielectric constant and a low dielectric loss tangent by combining a monofunctional cyanate ester compound and a polyfunctional cyanate ester compound (see Patent Reference 10).
Also described is a method of producing a poorly moisture-absorptive, flame-retardant cyanate ester-containing curable resin composition by adding a halogen-containing monofunctional cyanate ester thereto so as to lower the dielectric constant and the dielectric loss tangent of the composition (see Patent Reference 4). The patent reference describes cyanate esters in a broad range, but for keeping their flame retardance, an aromatic monofunctional cyanate ester having bromine as a functional group is an indispensable component, and no one could succeed in improving the flame retardance by the use of a cyanate ester resin alone. As described in the above, these bromine compounds may have a risk of generating corrosive bromine or hydrogen bromide through thermal decomposition.
Also provided are an aromatic cyanate ester compound having at least two rings bonding to each other via an unsaturated group-containing group (see Patent Reference 11), a fluorine-containing dicyanate ester (see Patent Reference 12), a method of using a phenol-novolak-type cyanate ester for attaining flame retardance (see Patent Reference 13), etc. In any case, however, an example of satisfying all of practicable low dielectric characteristics, flame retardance and heat resistance by the use of a cured product of a cyanate ester compound alone is not known.
In case where a cyanate ester is used in laminates for printed circuit boards, first it is dissolved in a solvent such as methyl ethyl ketone to prepare a varnish, and then this is impregnated into a glass cloth and dried to give a prepreg. When the steps are taken into consideration, the solubility of cyanate ester in solvent and the low volatility and the stability thereof in the working process are also important factors.
Patent Reference 1: JP-B 4-24370
Patent Reference 2: JP-A 2-286723
Patent Reference 3: JP-A 7-207022
Patent Reference 4: JP-A 6-122763
Patent Reference 5: JP-A 2000-95938
Patent Reference 6: JP-A 2003-128928
Patent Reference 7: JP-A 2003-128753
Patent Reference 8: JP-A 2003-128784
Patent Reference 9: JP-A 6-228308
Patent Reference 10: JP-A 6-49238
Patent Reference 11: JP-T 2002-531989
Patent Reference 12: JP-A 63-250359
Patent Reference 13: JP-A 2002-206048